This invention generally relates to the purification of sea water, brackish water, or contaminated fresh water, and is specifically concerned with a portable system and method for the bulk purification of raw or contaminated water by a freezing method wherein the pure water component of the raw contaminated water is frozen out. A continuous flow of the raw water is used to wash away the solid and gaseous contaminants from the surface of the pure water ice as it forms so that the contaminants do not become entrapped back into the ice solids.
Portable devices for the bulk purification of sea water or contaminated water are generally known in the prior art. Such devices are particularly useful in military situations where troops are stationed in remote areas of the world where non-potable raw water from the sea or a swamp or the like is the only water available. As the result of the demand that the military has historically had for the bulk purification of non-potable water, both the United States Army and Navy have been among the principal developers of mobile water purification devices. Thus far, the mobile bulk water purification devices developed by the United States military have been based on reverse osmosis technology wherein modules of semi-permeable membranes remove salt and other impurities by well known osmotic principals.
While such prior art water purification devices are capable of fulfilling their intended function, the applicants have observed a number of shortcomings associated with them. First of all, both the construction and operation of reverse osmosis water purification devices are expensive, for a number of reasons. To start with, considerable energy is required to overcome the osmotic pressure of the raw water. Additionally, an elaborate raw water pretreatment system is required to prolong the life of the expensive reverse osmosis membranes used in the device. The design of the pretreatment system depends on the type and amount of contaminants in the raw water, which requires a customized system of filters and other elements which in turn limits the versatility of the purification device. Additionally, the custom design work necessary to create the particular kind of pretreatment system necessitated by the particular contaminants in the raw water that the device will remove is time consuming, costly, and requires an extensive analysis of the raw water to be treated. Even with the best of pretreatment system designs, some degree fouling ultimately occurs in the reverse osmosis membranes, which of course necessitates their replacement. Presently the United States Army uses a trailer mounted Reverse Osmosis Water Purification Unit (designated by the acronym ROWPU) and the U.S. Marine Corp uses a skid-mounted ROWPU. The reverse osmosis modules of such ROWPU units must be replaced every few hundred hours on the average. The estimated replacement cost for the reverse osmosis modules used in a ROWPU having a capacity of 600 gallons per hour exceeds a hundred thousand dollars per year.
Bulk water purification systems that employ the principle of freeze purification are also known in the prior art. Such systems work on the principle that as water freezes, the growing crystals of ice displace and expel contaminants to the surface of the resulting ice body. However, in order to prevent the displaced contaminants from becoming entrapper within the ice crystals, many of these systems employ a direct freezing method which creates ice crystals that are dendritic in shape, which in turn gives them a relatively high surface area per volume of ice (i.e., greater than 100 cm.sup.2 per mL). As it is necessary to wash the resulting crystals in pure water in order to remove all surface contaminants before the crystals are melted down to create the resulting yield of purified water, the rinsing of these crystals causes a large percentage of the ice contained therein to melt. Such unwanted melting greatly lessens the resulting yield of purified water, and greatly increases the amount of energy needed to create a specific volume of pure water. Additionally, because the refrigerant comes into direct contact with the water in these methods, it is possible for the resulting product water to be contaminated with the refrigerant. Finally, the direct freezing methods used are relatively difficult to implement, and require the design and construction of specialized equipment which is heavier, bulkier and much more expensive than commercially available ice making machinery.
Accordingly, there is a need for a water purification system that is at least as effective as the ROWPU used by the United States military, but which is smaller, lighter, longer lived, more energy efficient, and more economical both to construct and to operate. Ideally, in military situations, such a system would not only be capable of desalinizing sea water, but would also be capable of removing biological chemical warfare toxins from any available water supply. Such a system should not require a custom built pretreatment system, nor require extensive water analysis and custom engineering to design, nor specialized freezing machinery. Finally, it would be desirable if such a portable bulk water purification unit was small enough and inexpensive enough to be used not only by the United States military, but also by campers, remote area residents, resort areas in which there was little or no natural supply of potable water, and in emergency situations (such as hurricanes and earthquakes) where the supply and distribution of potable water had become temporarily interrupted.